As one of the first cells to contact HIV, dendritic cells (DCs) are believed to play a central role in establishing infection and subsequent viral dissemination. All currently defined interactions between HIV and DCs rely upon the HIV Env. Previous work from our lab has demonstrated that HIV is capable of establishing robust interactions with DCs in an Env-independent manner and that the virus is capable of trans infection through this mechanism. This proposal seeks to define the DC receptor that mediates Env-independent binding and to investigate the role that this interaction has on HIV pathogenesis. Specific Aim I is to identify the DC receptor that mediates Env-independent HIV binding. Preliminary data suggests that HIV binds to a DC glycosphingolipid (GSL). In order to specifically identify the type of GSL that is acting as a DC receptor, two separate but complimentary approaches will be used. As GSLs are highly conserved between murine and humans, and murine DCs can bind and transfer HIV in an Env-independent manner, a murine DC model will be utilized. This approach allows for identification of the DC receptor through existing GSL knockout mouse models and species polymorphisms. In addition, primary human DCs will be treated with RNAi to target specific regions of the GSL biosynthesis pathway and subsequently test the effects on Env-independent binding and transfer. In specific Aim II, the way in which Env-independent binding impacts the fate of the virus will be characterized. Our lab has previously demonstrated that HIV traffics through a DC and reaches the T lymphocyte infectious synapse irrespective of the presence of Env. Using fluorescently labeled virions and deconvolution microscopy, this aim will define the relative contribution of Env-independent binding to efficient transfer of the virus. Identification of an Env-independent DC receptor is an important development in our understanding of HIV transmission. As DC interactions are particularly important in the establishment of early infection, an Env-independent DC receptor may be uniquely amenable to the efficient design of small molecule inhibitors that could serve as microbicides. In addition, this pathway may define a common mode of interaction between DCs and RNA viruses that bud from lipid rafts and share host-derived membrane compositions similar to HIV.